The present invention relates to a method of producing a preform for hydrostatic forming, and specifically, relates to a method of producing a plate stack preform useable for hydrostatically forming a product including a frame and a reinforcement which is disposed within the frame and has an X-shaped or cross shaped section.
Japanese Patent Application First Publication No. 2003-320960 describes a method of producing a frame member, for example, a side sill, a center pillar and a side roof rail, by a so-called hydrostatic forming process that is also referred to as a hydroform process or a hydrostatic bulge forming process. In the method of this related art, in order to provide a frame member with a reinforcement having an X-shaped or cross-shaped section from the viewpoint of rigidity, a four-plate stack preform is used. The four-plate stack preform includes two outer plates and a reinforcement stack between the two outer plates which is formed by two overlapping reinforcing plates joined to be in the form of a flattened X-shape or cross shape in section. The two reinforcing plates are welded to each other at a point corresponding to an intersection of the X-shape or cross shape of the reinforcement and welded to the two outer plates along opposed peripheral edges thereof. The two outer plates are welded to each other along opposed peripheral edges thereof. Upon hydrostatic forming, the four-plate stack preform is set in a die and supplied with a hydraulic pressure to thereby bulge the outer plates until the outer plates come into intimate contact with an inside surface of the die and the flattened X-shape or cross shape of the reinforcement stack is developed into the X-shape or cross shape of the reinforcement. Thus, the frame member with the reinforcement having the X-shaped or cross-shaped section is produced.